The present invention relates generally to a lighting device and, in particular, to a lighting device to be placed in front of a panel to illuminate the panel.
In order to improve the viewing of the non-emissive display, especially reflective type B/W and color display, one may place a light guiding panel on top of the display and adjacent to a light source in order to direct the light produced by the light source onto the viewing surface of the display. Typically, light is channeled into one or more edges of the light guiding panel. In U.S. Pat. No. 5,835,661, Tai et al. discloses a light guiding panel wherein a plurality of reflecting grooves are provided on the top surface of the light guiding panel in order to reflect the light introduced into the edges towards the display. As disclosed in U.S. Pat. No. 5,835,661, the reflecting grooves are uniformly distributed over the top surface of the light guiding panel. Accordingly, the illumination on the display is uneven such that the display section near the light source appears much brighter than the display section further away from the light source. This uneven illumination is more pronounced when the thickness of the light guide is reduced. The uniformly distributed grooves may also produce Moire patterns if the spacing of the pixels in the display is slightly different from the groove spacing. Using the light guiding panel, as disclosed U.S. Pat. No. 5,835,661, specular reflection of the illuminating light and ambient light from the lower surface of the light guiding panel further reduces the viewing quality of the display. In order to reduce this specular reflection, one may provide an anti-reflection coating, such as a thin film, on the lower surface. The anti-reflection coating is, in general, expensive because one or more additional steps must be carried out in order to apply the coating on the light guiding panel during the manufacturing process.
There are a number of known ways to provide light to a light guiding panel. For example, a cold cathodic fluorescent tube (CCFT) can be used as a light source and can be placed near the edge of light guiding panel to provide light into the light guiding panel. A CCFT typically consists of a glass tube, coated with phosphor on the inside of the glass tube, which is hermetically sealed and evacuated. When a high voltage is applied across the tube ends, plasma is formed to excite the phosphor for light production. The CCFT is relatively large as compared to the thickness of the light guide. It is especially bulky when it is used in a portable communication device such as a mobile phone. In U.S. Pat. No. 5,835,661, Tai et al. also discloses a linear light pipe which can be made sufficiently thin to be used as a light source to provide illumination to a display in a portable communication device. However, the linear light pipe, as disclosed U.S. Pat. No. 5,835,661, is not very efficient in that the light rays exiting the linear light pipe surface towards the edge of the light guiding panel are scattered in a random fashion. Thus, only a small portion of the light available from the linear light pipe is actually directed towards the viewing surface of the display.
Thus, it is desirable and advantageous to provide a method and a device for improving the front lighting of a non-emissive panel or display.
The first aspect of the present invention is to provide a method for improving lighting of a display having a top surface for viewing. The method comprises the steps of:
providing a light guiding panel substantially on the top surface of the display; and
providing at least one light source adjacent to the light guiding panel for providing light thereto, wherein the light guiding panel has a lower surface facing the top surface of the display, an upper surface, and at least one side edge for admitting a portion of the light provided by the light source through the side edge into the light guiding panel between the upper and lower surfaces, wherein the upper surface includes a plurality of grooves having dense-rare boundaries substantially facing the light source for reflecting part of the admitted light in a total-internal reflection fashion towards the display through the lower surface of the light guiding panel, and wherein the dense-rare boundaries have a distribution density which varies according to a distance between the dense-rare boundaries and the side edge.
Preferably, the distribution density increases with the distance.
Preferably, the lower surface includes a reflection reduction structure to reduce unwanted reflection of light from the lower surface towards the upper surface.
Preferably, the reflection reduction structure comprises a plurality of periodic grooves, wherein the periodic grooves have a pitch, which is smaller than half the dominant wavelength range of the light provided by the light source.
The second aspect of the present invention is a lighting apparatus for a display having a top surface for viewing. The apparatus comprises:
a light guiding panel located substantially on the top surface of the display; and
at least one linear light source located adjacent to the light guiding panel for providing light thereto, wherein the light guiding panel has a lower surface facing the top surface of the display, an upper surface, and at least one side edge for admitting a portion of the light provided by the linear light source through the side edge into the light guiding panel between the upper and lower surfaces, and the upper surface includes a plurality of grooves having dense-rare boundaries substantially facing the light source for reflecting part of the admitted light in a total-internal reflection fashion towards the display through the lower surface of the light guiding panel, and wherein the dense-rare boundaries have a distribution density which varies according to the distance between the dense-rare boundaries and the side edge.
Preferably, the linear light source includes:
a linear light pipe having a peripheral surface and at least one light-input end; and
at least one light emitter adjacent the light-input end for providing light thereto, and wherein the peripheral surface allows the light provided by the light emitter to transmit therethrough towards the side edge of the light guiding panel.
Preferably, the linear light pipe has a longitudinal axis substantially parallel to the side edge, and the peripheral surface has a cross section substantially perpendicular to the longitudinal axis, wherein the cross section includes a curved section adjoining a reflecting surface which is located away from the side edge of the light guiding panel in order to reflect light towards the side edge through the curved section.
Preferably, the curved section has a spherical-shaped section adjacent the side edge for focusing the light reflected by the reflecting surface towards the side edge.
Preferably, a plurality of scatterers are provided on the reflecting surface to scatter light towards the curved section.
Alternatively, the curved section joins the side edge such that the linear light pipe is an integral part of the light guiding panel.
The third aspect of the present invention is to provide a light guiding panel which comprises: a first surface, an opposing second surface and at least one side edge, wherein the first surface has a plurality of grooves having dense-rare boundaries substantially facing the side edge so as to reflect light admitted through the side edge into the light guiding panel between the first and second surfaces towards the second surface, and wherein the dense-rare boundaries have a distribution density which varies according to the distance between the dense-rare boundaries and the side edge.
Preferably, the light guiding panel further comprises a reflection reduction structure provided on the second surface.
Preferably, the reflection reduction structure includes a plurality of periodic grooves.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 1 to 8.